As is well known in the art, the examination and analysis of integrated circuits requires sophisticated sample preparation techniques and imaging tools. In the past, integrated circuits were generally constructed using aluminum for metal lines in each of the metal layers of the integrated circuit and tungsten for vias interconnecting the metal lines with components formed on a polycrystalline silicon layer. Since aluminum and tungsten can be selectively etched, integrated circuits could be deconstructed using selective etching techniques that permit the vias to be segregated from the metal lines, as will be explained below in more detail with reference to FIG. 1. Furthermore, modern integrated circuits generally require sophisticated imaging equipments such as a scanning electron microscope because components are frequently too small to be visible under an optical microscope. In order to distinguish vias from metal lines, it is therefore necessary to acquire images that show contrast between the vias and the metal lines. Tungsten and aluminum are readily distinguished in scanning electron microscope images.
Consequently, a prior art technique illustrated in FIGS. 1a-1d for preparing an integrated circuit die for imaging is commonly used to acquire tile images of a deconstructed area of interest of an integrated circuit die. FIG. 1a is a schematic cross sectional diagram of two metal layers of an integrated circuit die generally indicated by the reference 10. As is well known in the art, each metal layer is covered by an interlayer dielectric (ILD) 12 of a suitable material well known in the art. A metal layer N+1 is separated from the interlayer dielectric 18 on which it is deposited by a barrier layer 16, also composed of a suitable material well known in the art. The barrier layers 16, 22 prevent the deposited metal layers N+1, N from migrating into the interlayer dielectric 18, 24 onto which they are deposited. A metal line 14 of metal layer N+1 is connected to a metal line 20 of metal layer N by a via 26, which is also formed in a manner well known in the art. The barrier layer 16 that separates via 26 from metal layer N is conductive and provides an electrical connection between the via 26 and the metal line 20.
In order to acquire tile images of the integrated circuit 10, passivation layer 12, and any optional barrier material (FIG. 1a) is first removed using a wet or dry etching process or a chemical and/or mechanical polishing process to expose metal lines 14 of metal layer N+1. The integrated circuit die 10 is then placed on a precision stage of the imaging equipment, a scanning electron microscope for example, and tile images are acquired of the area of interest in a manner well known in the art. After the tile images of metal layer N+1 have been acquired, the metal layer N+1 is removed using, for example, a wet or dry etching process or a chemical and/or mechanical polishing process. The process is controlled to remove the metal layer N+1 while preserving the integrity of the vias 26, as shown in FIG. 1c. Thereafter, an etching solution is selected that will remove the barrier layer 16 as well as the interlayer dielectric 18 while leaving the via 26 intact. The results of that etching step are shown schematically in FIG. 1d. If the etching is carefully controlled, the via 26 remains intact and portions of the barrier layer 16r that are shielded by the via 26 and surround the via 26 remain after etching is complete. Thus metal lines 20 of metal layer N and the via 26 are exposed and tile images of the exposed via 26 and metal layer N are acquired in a manner well known in the art.
This prior art process can be referred to as a “bottom up” process because the vias are imaged in conjunction with the metal lines to which they are connected at their bottom ends. While this prior art technique works well for integrated circuits constructed using aluminum metal lines and tungsten vias due to the different etching characteristics of the two metals, integrated circuits are now being manufactured using copper metal lines and copper vias. This makes the prior art method very difficult to perform and complicates layout extraction, as will be explained below with reference to FIG. 2.
FIG. 2 is a reproduction of an image of a copper damascene integrated circuit prepared using the prior art process described above with reference to FIGS. 1a-1d. The image 30 was acquired using a scanning electron microscope. The integrated circuit die was prepared for imaging using a controlled etching process that removed the metal lines of metal layer N+1 and the interlayer dielectric 18 while leaving, to an extent possible, the vias 26. As can be understood by those skilled in the art, the etching process is difficult to control when the vias and the metal lines are made of the same metal. Consequently, some of the vias 26 are eroded and have an oblong shape in the image. As well, the copper lines 32 and the vias 26 are very similar in shade and it is not consistently clear to which metal line 32 a via 26 is connected. Circuit layout information is therefore difficult to extract and prone to errors.
There therefore exists a need for a method of preparing an integrated circuit die for imaging to permit a structure and layout of the integrated circuit to be extracted, regardless of metals used to construct the integrated circuit.